Question

what gives legal validity to letter?​

Answer 1

Answer:

It is valid against you - you cannot ever claim anything which is NOT specified there.

It is valid against the Company to the extent it does not refer to any other Policy document - if it does, then you can claim only for contravention of BOTH the Offer Letter and the Policy referred to.

That said, this is conceptual and theoretical - In the Indian legal context neither you nor the Company have the time &/or money to go to court for enforcement, as such it forms the amicable understanding framework - contravention results in immediate severance.